Whenever Dexter has his victim on the slab ready to be killed, he cuts them with a surgical knife and takes a blood sample. Obviously the knife is fake and the blood is fake but quite often you will see the knife touch the skin and the fake blood comes out, emulating a cut on the cheek.

Also worth taking a loom at this which talks about how some effects on Dexter are done
– LongshanksNov 30 '17 at 18:30

I remember a scene from NCIS where Abby 'open her throat' with one of those knife that 'draw blood' to get Gibbs attention (and to prove that the murder in the video was not a murder)...
– dnaMay 21 '18 at 7:00

2 Answers
2

It's a hollow knife - fake blood is pumped through and ejected when the fake blade touches the skin.

Or the blood is routed along the (very blunt) blade on the side away from the camera. If you're a right-handed actor, you can use this knife to slit someone's throat without the pipework and bladder being visible to the camera.

Blood

Meltzer actually created his own blood formula for the show, consisting of maple syrup, food coloring, peppermint oil, and Dawn dish soap.
Maple Syrup adds the thicker texture and cleans easier than corn syrup, peppermint oil repels the insects that are attracted to the syrup, and the Dawn soap makes the fake blood easier to clean out of clothes.

Blood Samples

So when Dexter collects the blood samples, the blood is the maple syrup blend, but when we see the case of samples the dried blood is actually furniture dye.

Cuts

Now, finally to your question.

In these scenes, Dexter uses a rigged knife, which is essentially a dull-blade knife that has a "blood pool" on one side (the knife below is the one used on set).

They have knives with blood pools on different sides to get cuts from varying angles.